1. Field of the Invention
The present invention relates to a method for recovering lead and zinc values contained in secondary scrubber sludges or muds from blast furnace gases.
2. Description of the Prior Art
The smokes produced during the manufacture of cast iron and steel cannot be discharged into the atmosphere except after having undergone dust removal treatment. The methods and equipment enabling this operation to be effected then ensure discharge to the atmosphere in accordance with the regulation standards, whilst the dusts, generally trapped in a wet medium, are extracted and recovered in the form of muds. These muds, called secondary scrubber muds, are generally constituted by elements of the blast furnace charge (iron ores, more or less reduced agglomerates, coke, and similars) and non-ferrous metals such as lead and zinc. They are, according to their iron content, either recycled after agglomeration, or abandoned for discharge into the open air. In the first case, the recycling increases the proportion of zinc and of lead of the ores charged into the blast furnace, which causes considerable disturbances in its operation through attack of the walls, the slowing down of the transit of the ore by clogging up, the presence of injurious vapors during casting and degradation of the quality of the cast iron and the steel. In the second case, the abandonment on slag heaps of noxious muds causes the pollution of rivers and of water-bearing layers.
The production of muds is in the vicinity of 2 to 3 kg of dry matter per ton of hematite cast iron and 5 kg per ton of phosphorous cast iron; the contents being variable from 3 to 25% of zinc and 1 to 9% of lead and confer on these muds the character of ores rich in these elements. Considering the European production of cast iron, the processing of these muds for the recovery of these metals is of high economic interest and contributes, as has already been indicated above, to the battle against pollution, to the improvement of the operation of iron and steel works and to the integral recycling of the residues.
The methods for recovering zinc and lead values from dusts of iron metallurgy plants comprise processing methods by the dry route and processing methods by the wet route.
Among the methods by the dry route, there may be mentioned notably:
the chlorination processing method according to NEUHAUS et al. (Stahl. Eisen. All., 78 (1958)pp 1662-1670) in which the use of MgCl.sub.2, CaCl.sub.2 and NH.sub.4 Cl in the agglomeration of the muds enables the recovery of zinc and lead values. For example, the addition of 3% of CaCl.sub.2 into a mixture to be agglomerated including 10% of blast furance gas scrubber muds enables the removal of 66% of the zinc and 90% of the lead.
firing and fusion of pellets in a 15 KVA furnace according to PINAEV A. K. [Stahl. (URSS) 11 (1972) pp 1056-1057] enables the obtaining of a 98% iron alloy and a recovery of 95% of the zinc.
reduction in a rotary furance by the method of MEYER et al [Stahl. Eisen. All., 96 (1976) p 1228-1233] or that of SUGASAWA et al. [Comm. Seminar on the utilization of prereducted materials in iron and steel making. Bucarest, 27-28 May 1976. CEE Steel/SEM/2/RI, 7 pages].
In Europe, outstanding research has been carried out in the use of the Waelz Berzelius furnace [E. GORY--Treatment of muds and dusts from iron metallurgy--Association Technique de la siderurgie francaise, Commission de l'environment EV SO--June 1975] for the recovery of the metals contained in zinc ores which are low grade and of small particle size and in zinc- and lead-containing residues (ashes and slags). It is observed however that the commercial benefits of this furnace can only be ensured if the input contents of lead and of zinc are sufficient and higher than 10%.
The reduction process in a vertical column of the New Jersey type ["Zinc and its Metallurgy" --Author: Centre Technique de zinc. Ed. CT 2-34, rue Collange 92307 LEVALLOIS PERRET 4th edition--June 1974].
Among the wet processing methods, may be mentioned notably:
the extraction of the zinc and the lead by means of ammoniacal solutions of ammonium carbonate (WAGNER et al. Recherche CECA No. 235 (1976),
extraction by means of sodium solutions (HISSEL et al. Recherche CECA No. 6252 12/2/235 1976).
The majority of the abovementioned methods are not of much interest for economical reasons.
On the other hand account should be taken, by way of prior art, of the following publications:
French patent application No. 77-25788, wherein a process is described enabling the selective removal of the zinc from the recovered dusts of iron metallurgy.
This process includes a complex series of operations and necessitates fine pulverization of all the dust to be processed and, without alluding to any granulometric cut, makes use of wet grading which must be carried out under vacuum. Moreover, it does not permit the separation of the lead and lead compounds at the same time.
DE-AS No. 1,170,648, in which a process is described for removing the dust by the wet route from the residual gases containing lead and zinc. This process ensures the selective separation by the hydrometallurgical route (leaching of the dusts with H.sub.2 SO.sub.4 and sodium solutions) of the lead and of the zinc.
DE-AS No. 1,533,070, in which a process is described for the treatment of metal-containing fine dusts. This process only involves ideas of granulometry to distinguish the various degrees of humidity to be respected, which is considered as the essential variable. This process brings into action also a prior mixing of the muds with finely divided solid reducing agents, and then, after adjustment to the typical and otpimal degree of humidity according to the granulometric spectrum of the dusts and/or the treated muds, drying in a straight oven and fusion in a reducing atmosphere of the agglomerated powders, thus producing volatilization of the lead and of the zinc.
U.S. Pat. Nos. 3,524,743 and 3,547,623 which, both, relate to processes for suitably agglomerating metal containing muds (by the addition of specific reactants before introduction into a suitable furnace) and specify what must be the conditions of reduction; the processes applied work by pyrometallurgy.
French patent application No. 76-02193, in which a process is described for treating powders containing metals (such as lead, zinc, iron and the like), which include the application of powders composed on the one hand of extremely fine particles ranging from 2 to 3 .mu.m and on the other hand coarse particles of about 1 mm and relies on conversion of these powders into a mud having well-defined concentration characteristics and itself intended to undergo classifying or hydraulic grading. Thus, it is proposed to put all of the extremely fine particles and coarse particles in the form of a mud having a solid concentration not exceeding 50% and, after having subjected the thus constituted mud to hydraulic classification, to collect, at the upper part of the hydraulic grading plant, a material enriched in zinc and in lead. According to this method, the overall concentration in solids of the mud and not the particle sizes of the solid particles is involved. Moreover, the obligation imposed of dealing with two groups of very highly differentiated solid particles is purely an idea, which does not correspond at all to reality; now, if this restriction is not respected, the process described in this document is inoperative and furthermore does not take into account of the state of the muds, which may be flocculated or not.
There has now been found a simple method which enables the production, from secondary scrubber muds of blast furnace gases, on the one hand, of a product having a higher content in lead and in zinc which can then be treated for the recovery of the lead and of the zinc, and on the other hand, of a residue having a lower content in lead and zinc which is recyclable for feeding the blast furnace.
It has in fact been found, unexpectedly, that the non-ferrous metals, such as zinc and lead, have a tendency to be concentrated in the fines when their content increases in said muds.
The method according to the present invention is defined as being a method of preconcentration by a physical route and essentially consists of subjecting blast furnace scrubber muds to particle size grading so as to concentrate the lead and the zinc contained notably in the fines, and in recovering the fraction containing the lead and the zinc, the other fraction being recyclable to the blast furnace. Basically, there is recovered, as a fraction containing lead and zinc, the fraction whose particle size is less than about 50 .mu.m and essentially in practice the fraction whose particle size is less than a value comprised between about 10 and 50 .mu.m.
The muds applied in the method of the invention are secondary scrubber muds from blast furnace gases, which are flocculated or not.
The particle size grading may be carried out in a single step or in several steps. For example, in one embodiment of the method according to the invention, it may be advantageous to subject the muds previously to a particle size grading by separation at a value comprised between about 80 and 150 .mu.m and then to continue with a particle size grading at a value comprised between about 10 and 50 .mu.m, preferably between about 20 and 50 .mu.m, of the fraction resulting from the prior step and possessing a particular size less than about 80-150 .mu.m.
This prior particle size grading is not essential in all cases; examination of the mineralogic and granulometric characteristics of the muds to be processed enables a decision to be made of the opportuneness of this separation at a particle size value between about 80 and 150 .mu.m.
The fraction of coarsest particle size thus obtained is processed for the recovery of solid material (decantation or filtration) which could then be recycled, after agglomeration, for feeding the blast furnace.
The particle size gradings may be effected by any suitable means. It is possible, for example, to carry out the particle size grading by sifting by means of a series of conventional sieves (+250 .mu.m; +140-250 .mu.m; +63-140 .mu.m; +37-63 .mu.m and 0-37 .mu.m).
These particle size gradings may also be effected by centrifugation in a static apparatus, such as by means of a cyclone or by cycloning. It has been found that the latter mode of particle size grading is as efficient with the muds obtained after the addition of flocculating agents as with naturally decanted muds. It results therefrom that the method of the invention may be applied without modification in presently existing mud recovery installations. Other methods of particle size grading may be used for the application of the method of the invention. It may be judicious, according to the selected separation method, to add to the flocculated muds coming from existing installations products causing the deflocculation and the dispersion of the constituent solid particles of the mud. By way of example of such a product, may be mentioned polyphosphate based products.
When the processing is by cycloning, it may be advantageous to subject the fraction of particle size lower than the particle size value of about 20-50 .mu.m, to a subsequent separation at about 10 .mu.m.
The term "cycloning" designated a single cycloning as well as successive cyclonings.
The fractions of particle size lower than the particle size value of about 20-50 .mu.m or of about 10 .mu.m constitute muds preconcentrated in zinc and in lead and may be used for the recovery of the zinc and of the lead, whereas the fractions of particle size higher than the particle size value of 20-50 .mu.m or of 10 .mu.m may be recycled for feeding the blast furnaces.
In a particularly preferred embodiment of the method according to the invention, blast furnace gas scrubber muds are subjected to the following steps:
(1) particle size grading by separation at a particle size value comprised between about 10 and 50 .mu.m after, if necessary, a prior separation at about 80-150 .mu.m;
(2) processing by attrition of the fraction having a particle size higher than the particle size value of about 10-50 .mu.m;
(3) particle size grading of the fraction resulting from step (2) at a particle size value comprised between about 10-50 .mu.m;
(4) recycling to the feed for the blast furnace of fractions resulting from steps (1) and (3) and having a particle size higher than said particle size value of about 10-50 .mu.m;
(5) recovery of the fractions having a particle size lower than the particle size value derived from steps (1) and (3) for the recovery of Zn and Pb values contained in these fractions.
The fractions obtained by step (5), which may be called "preconcentrated muds", are particularly suitable as a feed material for the Waelz-Berzelius furnace or for any other pyrometallurgic or hydrometallurgic process since their lead and zinc content is generally higher than 10% by weight.